Revisiting the cosmological evolution of X-ray selected blazars
with the Swift/BAT 70-month all sky survey data
Koyo Toda,1 Yoshiyuki Inoue,2 Yasuyuki Tanaka,1 and Yasushi Fukazawa1
1

Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526
2
Department of High Energy Astrophysics, Institute of Space and Astronautical Science,
Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510
E-mail(KT): toda@astro.hiroshima-u.ac.jp
AbstractBlazars constitute the most extreme class of active galactic nuclei. Recent multi-wavelength observations allow us to understand the emission processes in blazars. In contrast, the cosmic history of blazars isstill veiled in mystery. The keys to understand their evolution is the cumulative source count distribution
and the luminosity function with which we are able to obtain the trend of evolution and the peak of blazar
formation epoch in the universe. In this talk, we report our study on the cosmological evolution of blazars
utilizing the 70-month data from the Swift/Burst Alert Telescope (BAT) survey. Our sample comprises 41
flat-spectrum radio quasars (FSRQs) and 27 BL Lacertae (BL Lac) objects. Our sample size is a factor of∼ 2 bigger than that of the previous study with the 22-month Swift/BAT survey data (Ajello et al. 2009).
Although Ajello et al. 2009 reported the peak of the FSRQs density at z ∼ 4, we suggest that the FSRQs
density has a peak z < 4; that is, the redshift evolution of FSRQs is not stronger than the previous study.
Furthermore, we will discuss prospects for blazar detectability by future X-ray survey missions such as
eROSITA satellite.
Key words: AGN:Blazar — Luminosity function

1.

Introduction

Blazars constitute the most extreme class of active galactic nuclei (AGNs). Blazars have a relativistic jet that ispointing toward the line of sight and emit with radiationfrom radio waves to gamma-rays. Astronomers currently
recognize two types of blazars. Those are known as FSRQs and BL Lac objects. FSRQs have high luminosityand high redshift, and furthermore, its Equivalent widths(EW) of optical lines is greater than 5Å. In contrast, BL
lac objects have low Luminosity and low redshift, andfurthermore, its EW of optical lines is less than 5Å.
In previous study, the density of the gamma-ray
blazars peaks at z ∼ 1 − 2 in the history of the universe (Ajello et al. 2012). However, the density of X-ray
blazars peak are reported to be peaked at z ∼ 4 (Ajelloet al. 2009).
In the previous study, the number of X-ray blazar sample, which comprises 26 FSRQ samples from Swift/BAT
22-month catalog, is less than that of gamma-ray blazarsample, which comprises 168 FSRQ from the Fermi firstyear catalogue. Then, it is important to derive number
density of FSRQs utilizing more X-ray blazars.

Therefore, the purpose of this study was to explore thecosmological evolution of blazars utilizing the 70-month
data from the Swift/BAT survey which is the newest
hard X-ray all sky survey map.
2. Sample Selection
In this work, we used the Swift BAT 70-Month Hard
X-ray Survey data (Baumgartner et al. 2013). This survey comprises 1211 X-ray objects which was detectedby Swift/BAT within 70 months. In order to securely
identify and classify BAT sources as blazars, we relied
mainly on the blazar catalog (BZCAT) (Massaro et al.
2009) which contains only bona fide blazars. As a result,
our blazar sample comprises 41 FSRQs and 27 BL Lacobjects and the size of our sample is a factor of ∼ 2 bigger than that of the previous study with the 22-month
Swift/BAT survey data (Ajello et al. 2009).
3. Result3.1. logN-logS distribution
We derived logN-logS distribution. This distribution is
generally expressed as N (> S) = AS −β ,where S is asource flux. If the source population is uniformly distributed, a β =1.5 is expected.
The logN-logS distributions we derived is shown in Figure 1 and Table 1. This logN-logS distribution indicatesthat FSRQs is positive evolution and BL Lac obejacts isnegative evolution.

3.2.2. Number density of FSRQ
Based on 2powMPDE and 1powMPLE model, we can seethat the peak of the FSRQ density at z < 4. This suggests that the redshift evolution of FSRQs is not stronger
than the previous study which used 22month Swift/BAT
data.

Fig. 1. LogN-LogS distribution of FSRQs (left) and BL Lacs (right).
The red line shows the result of previous study (Ajello et al. 2009)and black line shows the result of this study.

Table 1. The expected number of blazar detected by eROSITA satellite

Sample
FSRQs
BL Lac objects

βa1.87 ± 0.07
1.32 ± 0.13bβ22month2.077 ± 0.269
1.694 ± 0.316

Note.
a Best-fit exponent of the logN―logS distribution (e.g., N (> S) =AS −β )b Best-fit exponent of the previous study (Ajello et al. 2009)

Fig. 2. Number density of FSRQs. The blue line are derived fromthe [15―55keV] LF (Ajello et al. 2009), the black line are derived
from the gamma-ray LF obtained by Fermi/LAT (Ajello et al.
2012) and the red line are derived from the X-ray LF obtained bythis work.

4. Blazar detectability by eROSITA satellite
From our logN-logS, we can estimate the expected number of blazers detected by eROSITA satellite. The results
are given in Table 2
Table 2. The expected number of blazar detected by eROSITA

3.2. Luminosity function of FSRQs
The luminosity function (LF) is defined as the number
objects per unit comoving volume and per unit luminosity interval. The simple scenarios of evolution are pureluminosity evolution (PLE) and pure density evolution(PDE). We modeled the LF with eight widely used functions and applied the maximum likelihood method andKolmogorov-Smirnov (KS) test to derive the LF of FSRQs.
3.2.1. KS-test
In this work, 3 LF-models was not rejected:1powPDE
model, 2powMPDE model and 2powMPLE model.
These LF-models are listed below.

Observationtime
4 yearsurveySensitivity(erg/s/cm2 )2 × 10−13@2 − 10keV

FSRQs

BL LacObjects

13900

1900

Note. We assumed that logN-logS slope does not change in low
flux range.

5. Conclusion
We suggest that the evolution of blazars is not stronger
than the previous study (Ajello et al. 2009)) and the peakof the FSRQs density at z < 4 . We will estimate thecontribution of blazars to the cosmic X-ray backgroundoutside the energy range band of this survey using LF.
References
Ajello, M. et al. 2009, Apj, 699, 603
Ajello, M. et al. 2012, Apj, 751, 108
Baumgartner, W. H. et al. 2013, ApJS, 207, 19B
Massaro, E. et al. 2009, A&A, 465, 691